I'd like some advice on jewelry I can wear out and about when I'm presenting as a man. Preferably something that wouldn't get too many looks on a 40 year old.
Simple gold or silver chains are an obvious choice. Bead mala bracelets too.
Earrings are great. Lobe piercings are among the easiest to manage. And you can change jewelry often to get the right level of masc/femme you're looking for (e.g., studs, dangly). I find rose gold more femme than yellow gold, but finding the right jewelry colour for your skin tone (yellow gold, rose gold, silver or white gold) also looks femme. I like basic "huggies" earrings. They're feminine without being obviously feminine, and they're very low maintenance - easy to sleep with, never have to take them out. Once you get comfy with your lobes pierced, try a second set of lobe piercings :P
Basic (e.g., black rope) anklet is an option.
I'd like to find a choker necklace that can be worn in my he/they mode
I don't wear jewelry, generally speaking. And there appear to be cultural norms around jewelry, something like fine, ornate pieces being more fem, and bulky, simple pieces being more masc. As above, e.g. some earrings a simple hoop, not to big, or even one in each lobe. A necklace that had a more solid chain, a bracelet or ring that is more bulky, substantial, maybe as I'm thinking of it, that it represents wealth especially if it's gold or silver. And other materials like carbon fiber, steel, copper as they are tough and ultilitarian, which are for no particular reason associated with masc. Probably there's some stuff about some jewels being more fem, others more masc. And I'll reiterate, wear what you want to wear, what makes you feel good.
So I'm just a cis dude scrolling by but it looks like you want more "generally masculine" jewelry suggestions?
I've not seen anyone suggest signet rings in particular. They can be smaller or these great big chunky statements on your finger. I'm a particular fan of bronze because the best signets are always bronze. You can get custom engravings and with or without jewels, there's so many options.
Signet rings are sharp for any gender and can really make a statement. They're traditionally worn on the little finger, but I've always liked them on the fourth or middle finger, bold as brass.
If red works well with your style, check out carnelian signet rings. They are made with a bunch of cool designs.
depending on where you live / what is culturally acceptable, you could probably just wear whatever you like - gender non-conforming styles are something even cis-men explore, esp. since the 1960s in the U.S. when feminine hippie styles (long hair, flowy clothes, floral patterns, etc.) took on anti-war significance.
EDIT: you have to remember, cis people are truly clueless, most people won't question your gender as a man even with jewelry, feminine behavior, etc. - many people will even think there were no signs when you had been feminizing in many ways
EDIT2: some examples from my own life, I wore women's clothing like skirts whenever I could growing up, yet my parents, friends, and other family never thought I might be trans; my step-mother even gave me her hand-me-down jeans to wear and this was considered reasonable / normal. When I transitioned, everyone was shocked, my mother told me there were no signs growing up. I think feminine traits on a man just reads as "gay", so just remember your fear that you will be outed as trans based on feminized things like jewelry might be outsized, others almost certainly aren't thinking that even if it should be obvious.
Cishet but one thing i didn't see mention is just some kind of non metalic bracelet. In the past I've done a braided bracelet. Someone might ask if it has any special meaning but it won't ever be questioned beyond that. If you want to get an idea of what i mean just look on amazon for "braided bracelet for men" and there's a bunch of ideas. If you have a loved one you can always ask for something like this as a gift for that special meaning. Or just get one that you like the look and/or think would look good.
I'm also adding earrings, they were basically one of the very first things I did when I started transitioning. They've become relatively common on guys, and small studs won't stand out much at all, but they're something I notice every time I look in the mirror and make me feel very femme. Plus, you can very much up how much they stand out as you get more comfortable with presenting more femme. Plus, I work with my hands a lot and can't wear anything dangly/metallic at work, and I just generally find a lot of jewelry annoying/uncomfortable to wear, but I don't notice my earrings at all except when I see them
You're probably right that it wouldn't stand out. The only comments I'd get would be 'so, you're having a mid life crisis huh.' Something like that....